1. Technical Field
This invention relates to cylinder liners for use in internal combustion engine applications.
2. Related Prior Art
Cylinder liners are often employed in heavy duty diesel engine applications to line the bores of the engine block, in which the pistons reciprocate. The liners are often made of gray cast iron. The gray iron material has a graphite flake structure which provides good wear and toughness characteristics. The gray iron liners are typically heat treated by either a quench and tempering process or induction hardening. The quench and temper heat treatment produces a through-hardened martensitic microstructure in which the graphite flakes are dispersed. Induction hardening involves locally heating the inner wall surface of the liners followed by a rapid quenching to provide a case hardened skin that has primarily a martensitic microstructure in which the graphite flakes are dispersed.
For many engine applications, such heat treated gray iron liners exhibit sufficient strength and wear properties to provide good service. However, recent advancements in diesel engine performance has placed ever-increasing demands on the liners, and the demands of next generation diesel engines are expected to continue to a point where the conventional gray iron liners may not provide adequate services. The increased efficiency and power of the engines exposes the liner to higher and higher operating temperatures and pressures beyond the limits for which these traditional liners were designed to operate under. In addition to exhibiting acceptable levels of strength and wear properties, the liners for such engines must also exhibit good dimensional stability. When heated to an elevated operating temperature, the liners naturally undergo a certain amount of thermal expansion, which is recoverable once the liners cool. However, if the operating temperatures are high enough and the exposure time long enough, such liners can undergo phase transformations of the material which result in permanent irrecoverable thermal growth particularly near the upper end of the liners which are directly exposed to the combustion environment. In designing a liner for a given engine application, such anticipated thermal expansion and thermal growth must be accounted for. While good performance of the engine calls for a close fit between the liner and block, a high level of thermal growth requires adequate space for the growth of the material. Since closely fit liners are often restricted by the block against radially outward movements, the liners are caused to grow inwardly, reducing the diameter of the inner wall against which the piston runs. Under such conditions, the piston rings and shaft can wear prematurely as can the liner. Such anticipated growth xe2x80x9cshrinkagexe2x80x9d of the inner running surface of the liners would have to be accounted for in designing an engine, and thus the inner diameter of the liners would have to be sufficiently oversized, which is counterproductive to engine performance under normal operating conditions before such extreme conditions are encountered.
One known approach to addressing the problems associated with traditional liners in high performance applications has been to control the solidification of the gray iron during casting in such manner as to achieve an as-cast bainitic microstructure. While such a liner exhibits good strength, wear and dimensional stability under the extreme operating conditions, such critical control over the casting operation adds cost and complexity to the manufacture of cylinder liners.
Cylinder liners constructed according to the invention overcome or minimize the limitations of the conventional gray iron liners.
A method of making a cylinder liner according to the invention includes first casting a cylinder liner structure from gray iron material and thereafter austempering the liner to develop a bainitic microstructure.
According to a further preferred feature of the invention, the cylinder liner structure is maintained at the austempering temperature during heat treatment for a time sufficient to develop a dimensionally stable bainitic microstructure that exhibits a comparably low, acceptable level of thermal growth when exposed to extreme operating conditions of 450xc2x0 F. operating temperature for 20 hours.
According to still a further preferred aspect of the invention, the cast gray iron liner structure is maintained at the austempering temperature for at least about 5 hours to achieve a coefficient of thermal growth of less than about 0.3xc3x9710xe2x88x926 in/inxc2x0 F. at 450xc2x0 F.
Austempered gray iron cylinder liners constructed according to the invention have the advantage of exhibiting good strength and wear characteristics along with excellent thermal growth properties, making such liners well suited for the next generation high temperature, high performance engine applications.
The invention has the further advantage of achieving such properties by utilizing conventional low cost casting technologies coupled with controlled austemper heat treatment. There is no need to take such special care during casting to develop a particular as-cast microstructure. During the austempering heat treatment cycle, the cast liner structure is first heated to an elevated temperature to austentize the material and thereafter quenched to the austempering temperature at a rate and for a time sufficient to develop the desired dimensionally stable low thermal growth bainitic microstructure.